This invention concerns a guided-wave photoreceptor.
It particularly applies to the field of fiber-optic telecommunications, especially to coherent communications systems with polarization diversity.
Coherent transmission systems on single-mode optical fiber are currently undergoing extensive development both for very long-distance communications and for distribution.
In a coherent connection, the receptor (homodyne or heterodyne) makes it possible to detect an incident optical signal by mixing it with an optical wave from a local oscillator which is a perfectly monochromatic laser.
FIG. 1 shows the receptor 2, as well as an optical coupler 6 with two inputs and two outputs.
One input receives the signal S, and the other input receives the optical wave that comes from the local oscillator 4.
The wave that comes from the oscillator is mixed with the signal S using the coupler 6 whose two outputs are connected to the receptor 2 to supply it with two optical signals resulting from the mixture.
One of the delicate points of a coherent connection is that it is impossible to control the optical polarization of the signal S during transmission.
Due to the variable environmental conditions (temperature, torsion, . . . ) in which the single-mode optical transmission fibers must operate, which are difficult to control, the state of polarization of the signal S which arrives at the input of the coupler 6 fluctuates randomly.
This causes fluctuations in amplitude in the coherent detection.
Then it is necessary to incorporate into the receiving system (which includes the receptor 2, the local oscillator 4 and the optical coupler 6) a polarization control system (not shown in FIG. 1) which is placed before or after the optical coupler 6.
The photoreceptor includes photodiodes (not shown) which are respectively supplied with preamplifiers (not shown) that are hybrid-mounted or integrated.
But the tendency is to use hybrid mountings of pairs of standard equilibrated photodiodes.
For obvious questions of cost and overall performance of the connection, an attempt is made to put all the functions of the receiving system (coupler, polarization control system, photodiodes, preamplifiers and even the local oscillator) on the same substrate.
But such integration poses problems, particularly because a material suitable for making photodiodes for the optical wavelength considered is not necessarily a material suitable for microelectronic components, such as preamplifiers.
It would therefore be of interest to find some solutions that would make it possible to simplify the receptor system while offering advanced possibilities for micro-optical electronic integration with acceptable performance.